In one aspect, the invention relates to a magnetic shielding member. According to another aspect, this invention relates to a magnetically shielded drilling apparatus. According to yet another aspect of the invention, the invention relates to a method of drilling core specimens from a borehole core wherein the borehole core is effectively shielded from ambient magnetic fields.
During the drilling of a borehole in the search for oil or other minerals, borehole core samples are typically cut from the formations being traversed and are removed to the earth's surface for examination. Various valuable information can be obtained from such borehole cores via magnetic analysis, such as by paleomagnetic analysis wherein the natural remanent magnetism in the borehole core is detected. Natural remanent magnetism is that magnetism which was imparted to the rock by the earth's magnetic field as the rock formed. Probably the most important information obtainable by magnetic analysis of borehole cores is the dip and strike of fractures and of bedding planes in the core. Such information is determined by first determining the direction of the natural remanent magnetism in the borehole core by paleomagnetic analysis. From the magnetic direction, the original orientation of the borehole core in the earth can be determined. Once the original orientation is known, the correct strike and dip of fractures and of bedding planes in the core can be determined by observing such planes. Alternatively, other magnetic analysis techniques could be employed, such as those wherein a magnetic field is imposed artificially on the core before its removal from the surrounding subterranean formation.
It is known that borehole core samples are sensitive to ambient fields, such as the earth's, and can thus take on magnetic overprints resulting from exposure to such ambient fields. Obviously, such overprints can interfere in magnetic analysis of the core. Thus, magnetic analysis of borehole cores is frequently done in a magnetically shielded environment.
Magnetic overprinting from ambient fields has also been found to be a problem during drilling of a borehole core before a magnetic analysis procedure. Typically, small core specimens are drilled from the borehole core, such specimens being subsequently analyzed as discussed above. According to one prior method which attempts to compensate for possible magnetic overprinting during drilling, two core specimens on opposite sides of the borehole core are drilled therefrom. Such core specimens are sometimes referred to as antiparallel specimens. Overprinting in the antiparallel specimens can be cancelled out during magnetic analysis. It should be noted however, that according to such a technique two core specimens are required for an effective analysis, resulting in increased analysis time and cost.